1. Field of the Invention
The present invention is directed to the fabrication of II-VI alloy films, such as HgCdTe, on silicon for use in infrared (IR) detectors and detector arrays. More particularly, the present invention is directed to the growth of epitaxial II-VI(111) alloy films, such as HgCdTe(111), over large Si substrates for use in IR detectors and detector arrays.
2. Description of Related Art
Conventional HgCdTe IR detectors and detector arrays require epitaxial HgCdTe. CdTe and CdZnTe single crystals are the most widely used substrates for epitaxial growth of HgCdTe. CdTe and CdZnTe single crystals, however, are expensive and difficult to produce with large area. CdTe and CdZnTe substrates are also fragile and difficult to handle. Accordingly, other materials such as silicon and GaAs have been suggested as alternative substrates for HgCdTe. Silicon is especially promising because of low cost, large area, and good mechanical strength of silicon substrates. Additionally, the use of silicon substrates enables the prospect of fabricating monolithic focal plane arrays combining HgCdTe detectors with silicon integrated circuits. The silicon integrated circuits can be used for signal processing. Typically, a CdTe buffer layer is grown first on these alternative substrates, followed by the growth of the HgCdTe layer.
The use of silicon substrates for depositing HgCdTe is not without problems. Thermal mismatch between CdTe and Si produces additional strain to the CdTe buffer layer which is already strained. The large lattice constant mismatch between HgCdTe and Si results in the formation of a high density of threading dislocations in the II-VI composite film comprising the HgCdTe formed on the CdTe buffer layer. Additionally, the difference in coefficient of thermal expansion between HgCdTe and Si produces strain in the II-VI composite film. The strain develops as the structure consisting of the II-VI composite film formed on the Si substrate is cooled from elevated growth temperatures.
The use of Si as a substrate for HgCdTe infrared focal plane arrays (IRFPA's) is primarily motivated by: (1) the need to overcome the thermal cycle reliability limitations of hybrids formed by indium-bump bonding of HgCdTe array on bulk CdZnTe substrates to Si readout chips, and (2) the need to reduce system costs for implementation of large-area arrays.
For HgCdTe growth on CdTe and CdZnTe by liquid phase epitaxy (LPE), CdTe(111) and CdZnTe(111) are highly preferred over other crystallographic orientations. Heteroepitaxial growth of (111)-oriented CdTe on Si and GaAs substrates has been demonstrated in prior art. By (111)-oriented CdTe is meant CdTe(111). As used herein, CdTe(111) refers to epitaxial grown CdTe crystal having a surface that is a CdTe(111) face or CdTe(111) plane.
To perform heteroepitaxial growth of (111)-oriented CdTe or (111)-oriented CdZnTe on Si or GaAs substrates, vapor-phase techniques have been employed. In particular, molecular beam epitaxy (MBE) and metalorganic chemical vapor deposition (MOCVD) have been employed. Heteroepitaxial growth of (111)-oriented CdTe and (111)-oriented CdZnTe on Si or GaAs substrates by vapor-phase techniques, however, has historically been plagued by twin formation.
CdTe crystal comprises Cd and Te atoms arranged in a lattice having a zincblende structure (or sphalerite structure). CdTe can be visualized as being composed of a stack of close-packed planes. These closed-packed planes are parallel to the (111) plane of the zincblende structure, i.e., the CdTe(111) plane. Each close-packed plane comprises of a double sheet of atoms. Each of these close-packed planes possesses an identical arrangement of atoms as seen from the [111] direction. Each close-packed plane, however, is different from the two adjacent close-packed planes in that the atoms in each of these three planes are shifted with respect to the other two planes. All the atoms in each of the planes are shifted by a constant distance within the plane. These three different planes, denoted, e.g., A, B, C, can be visualized as comprising the CdTe crystal.
Two equivalent isoenergetic sequences of the closed-packed planes are possible: ABCABC and the reverse one, namely, BACBAC. A twin is formed when these two sequences are stacked on top of each other or if these two sequences nucleate side by side. The crystalline ordering of the twin is the mirror image of the original crystal.
X-ray analysis can be used to reveal the presence of twinning in a crystalline sample. An x-ray diffractometer, for example, can be employed to diffract x-rays from the crystalline sample. These diffracted x-rays are detected with a detector.
X-rays incident on a crystalline sample are diffracted in different directions. The x-rays diffracted from the crystalline sample form a pattern of reflection spots that comprise the Laue pattern. The Laue pattern may comprise symmetrically and/or asymmetrically oriented reflection spots. The Laue pattern comprises a series of reflection spots characteristic of the particular crystallographic orientation of the surface of the crystalline sample. Extra reflection spots observed in the Laue pattern indicate the presence of twins. Thus, the presence of twins in a crystalline sample can be revealed by the measurement of appropriately selected x-ray reflections.
More detailed information about the crystalline structure of the crystalline sample can be gained using different diffraction techniques. In particular, measuring the linewidth of symmetric reflection spots, or reflections, can be used to probe the crystalline quality. An example of such a symmetric reflection is the (333) reflection., i.e., the (333) Bragg reflection. Typically, the strength of the reflection at the detector of the x-ray diffractometer is measured as the sample is tilted or rocked. An x-ray rocking curve is produced by plotting of the strength of the reflection measured at the detector as the sample is rocked. The crystalline quality of the crystalline sample is assessed from the width, e.g. full width at half maximum (FWHM), of the x-ray rocking curve. Narrow diffraction lines indicate better crystallinity. Subgrain structure, dislocations, inhomogeneous strains, bending, and other extended defects contribute to the linewidth.
The theoretical width of the (333) reflection for a perfect crystal of CdTe is 4.7 arc-seconds. The theoretical width of the (333) reflection is much smaller than the width of the (111) reflection, which is 20.7 arc-seconds; see e.g., M. O. Moller et al, "Theoretical x-ray Bragg reflection widths and reflectivities of II-VI semiconductors", Journal of Applied Physics, Vol. 72, No. 11, pp. 5108-5116 (Dec. 1, 1992). Accordingly, linewidth measurements of the (333) reflection provide a better signal-to-noise ratio than linewidth measurements of the (111) reflections for use in assessing crystal quality.
As described above, twins are common defects in layers of CdTe(111) and HgCdTe(111) grown from the gas phase by MBE and MOCVD.
Twining in CdTe(111) and CdZnTe(111) films have been observed to be a problem for vapor-phase growth on GaAs substrates; see, e.g., A. Raizman et al, "Twin microstructure and effective particle size in (111)CdTe and ZnTe grown by metalorganic chemical vapor deposition," Journal of Applied Physics, Vol. 67, No. 3, pp. 1554-1561 (Feb. 1, 1990) who report twin formation in CdTe(111) which is grown on GaAs. See also, e.g., S. M. Johnson, et al, "X-ray diffraction analysis of Heteroepitaxial Cd.sub.1-y Zn.sub.y Te on GaAs", Material Research Society Symposium Proceedings, Vol. 144, pp. 121-126 (1989), who document twins formed in CdZnTe(111) which is grown on GaAs(111). D. J. Olego et al, "Optoelectronic properties of Cd.sub.1-x Zn.sub.x Te films grown by molecular beam epitaxy on GaAs substrates", Applied Physics Letters, Vol. 47, No. 11, pp. 1172-1174 (Dec. 1, 1985) and R. D. Feldman et al, "Growth of Cd.sub.1-x Zn.sub.x Te films by molecular beam epitaxy", Applied Physics Letters, Vol. 49, No. 13, pp. 797-799 (Sep. 29, 1986) also report the growth of CdZnTe(111) on GaAs, in particular, GaAs(100); however, twinning is not discussed.
A Si(001) surface has been utilized as a starting substrate in prior art attempts to deposit CdZnTe(111) and CdTe(111) films on Si. Twins are reported to be formed in CdZnTe(111) which is grown on a structure comprising GaAs(100) formed on Si(100); see, e.g., S. M. Johnson et al, "Heteroepitaxial HgCdTe/CdZnTe/GaAs/Si materials for infrared focal plane arrays", Material Research Society Symposium, Vol. 216, pp. 141-146 (1991).
CdTe is also deposited directly on the Si(001) surface; see, e.g., (i) R. Sporken et al, "Molecular-beam epitaxy of CdTe on large area Si(100)," Journal of Vacuum Science and Technology B, Vol. 9, No. 3, pp. 1651-1655 (May/June 1991), (ii) R. Sporken et al, "Current status of direct growth of CdTe and HgCdTe on silicon by molecular-beam epitaxy," Journal of Vacuum Science and Technology B, Vol. 10, No. 4, pp. 1405-1409 (July/August 1992), and (iii) Y. P. Chen et al, "Structure of CdTe(111)B grown by MBE on misoriented Si(001)," Journal of Electonic Materials, Vol. 22, No. 8, pp. 951-957 (1993), who report the growth of twin-free CdTe(111) on vicinal Si(001). As used herein, the term "vicinal Si(001)" refers to an epitaxial grown Si crystal having a surface that is a plane having an orientation near to, but not including, the exact Si(001) plane. To achieve twin-free CdTe(111) growth, CdTe was formed on a vicinal Si(001) plane which was tilted more than 8.degree. away from the exact Si(001) plane. Forming CdTe on a vicinal Si(001) plane having such large misorientations away from the exact Si(001) plane, however, results in extremely poor CdTe x-ray rocking curve widths. These x-ray rocking curve widths are greater than 600 arc-seconds. Forming CdTe on vicinal Si(001) planes having smaller misorientations away from the exact Si(001) plane are reported to result in an improvement in x-ray rocking curves. These x-ray rocking curve widths are reported to be as low as 140 arc-seconds. For these smaller misorientations, however, the CdTe(111) epitaxial layers, or epilayers, are heavily twinned. Chen et al also report that by subjecting the CdTe(111) film having x-ray rocking curve widths of 140 arc-seconds to a post-growth thermal anneal, then the x-ray rocking curves widths are reduced to 100 arc-seconds and the CdTe(111) film exhibit no twins in the top portion of the CdTe(111) film. However, TEM measurements, which are localized to the top of the CdTe(111) film, are used by Chen et al to establish the absence of twinning at the top portion of the CdTe(111) film. In contrast, x-ray measurements which would penetrate throughout most of the CdTe(111) films would show buried twins. Additionally, a post-growth anneal is required to reduce the x-ray rocking curve width.
Accordingly, the choice of a vicinal Si(001) surface for deposition of CdTe appears to result in a significant problem with obtaining a CdTe(111) epilayer having both (1) low twin density and (2) high crystalline quality.
As such, no effective technique is presently available for providing twin-free vapor-phase growth of (111)-oriented epitaxial II-VI alloy films that also provides high crystalline quality. By (111)-oriented epitaxial II-VI alloy films is meant epitaxial grown films comprising II-VI materials which have a surface that corresponds to the (111) face of a zincblende structure. In particular, at present, no technology has been demonstrated for depositing non-twinned CdZnTe(111) epitaxial layers or epilayers on Si substrates.
Thus, there remains a need for a method of forming epitaxial films comprising (111)-oriented II-VI materials on Si substrates which simultaneously achieves (1) low twin density and (2) high crystalline quality.